It has already been proposed to produce matrix sensors in which the resolution and sensitivity can be modified as chosen by the user by grouping a plurality of neighboring pixels. For a better resolution, the pixels are read independently of one another. For better sensitivity at the cost of lower resolution, the pixels are grouped and supply a single signal for each group of pixels. The adjacent pixels can be grouped 2 by 2 or 4 by 4 or more generally m by n, m and n being two integers, at least one of which is greater than or equal to 2.
The grouping of pixels (or “binning”) has been used in CCD (charge transfer device) technologies, where it is particularly effective because it allows photogenerated charges to be added by simple charge transfer. In active pixel MOS technologies, it is more complicated because the output signal from the pixels is a voltage; if the voltage is digitized and a plurality of digital signals are added in order to group pixels, the noise is likewise added.
However, even in active pixel MOS technologies, charge transfer structures have been proposed that allow unlimited grouping, in analog form, of photogenerated charges in a plurality of pixels. Charges are grouped by sharing the charge storage nodes of adjacent pixels before converting the charges into voltage; the disadvantage of the proposed devices is that this complicates the reading of the individual pixels when there is no wish to group them; moreover, these devices exhibit a lower charge/voltage conversion factor when the pixels are grouped than when they remain separate.
By way of example, the patent GB2474014 has proposed an architecture allowing conditional sharing of the charges between two pixels by means of a transistor that connects or does not connect the charge storage nodes of two adjacent pixels. The capacitance of the charge storage node greatly increases when this sharing is effected, which reduces the charge/voltage conversion factor.
Finally, the patent U.S. Pat. No. 6,452,153 has proposed charge sharing within the framework of a pixel that does not have an isolation transistor between the photodiode and a charge storage node and that, instead of this, has what is known as a “skimming” transistor that is DC-biased. This patent uses an architecture with a grouping transistor that is arranged so as to connect the outputs of a plurality of photodiodes to the read transistor of a single one of the pixels. Although the charge/voltage conversion factor remains the same when the pixels are separate and when they are grouped, the solution described in this patent has a major disadvantage that is low transfer efficiency because the charges on the photodiode that is furthest from the grouping pixel have trouble in reaching the read circuit of this pixel. The untransferred charges would appear in the next image, giving rise to a delay phenomenon that damages the image. Moreover, this structure has very little efficiency at low signal level because it works by skimming charges above a certain level. The grouping of charges is of particular interest at low light level.